In order to meet the demand for high clock frequencies of future computers, the integration density of current semiconductor devices as, for example, chips and multi-chip modules (MCM), in which a number of semiconductor chips are mounted on a common substrate generally in the face-down state, is continuously being increased. As a consequence, there arises the problem of increased heat dissipation due to the increase in power density.
Common cooling approaches include air cooling, as for example disclosed in the U.S. Pat. No. 5,471,366, which describes a multi-chip module and a fabrication process thereof. The multi-chip module comprises a plurality of semiconductor chips mounted on a common substrate and a plurality of thermally conductive blocks attached to the semiconductor chips. A resin package body encapsulates the semiconductor chips and the thermally conductive blocks together with the substrate. The resin package body furthermore has an upper surface flushing with the upper surfaces of the thermally conductive blocks. A heat sink carrying heat radiation fins is mounted onto the upper surface of the resin package body in such a way that a thermally contact is established between the heat sink and the upper surfaces of the thermally conductive blocks. Thus, heat dissipated from the semiconductor chips is transferred to the heat sink via the thermally conductive blocks and is radiated to the surrounding air from the heat radiation fins.
One disadvantage of this multi-chip module is that the heat transferred from the semiconductor chips to the air has to pass three boundary layers, so that the heat dissipation efficiency is reduced. Besides, the reworkability in case of defects or insufficient thermal connections is restricted due to the encapsulation of the thermally conductive blocks, the semiconductor chips and the substrate with the resin package body. Furthermore, in general air cooling approaches are quickly reaching their limits. As a consequence, alternative cooling concepts have to be pursued.
Other known cooling techniques include liquid coolants, which will be crucial for future midsize and portable computers in particular. The disadvantage of these cooling methods consists in the connection of liquid coolers to the surfaces of the semiconductor chips. In order to level out roughness and warp, a thick thermally interface between the chips and the liquid coolers is required which causes a high thermally resistance that deteriorates the cooling performance.
One of the currently used cooling methods is the so-called jet impingement cooling. The idea behind this cooling method is to spray a cooling fluid through nozzles directly onto the backside surface of the semiconductor chips in order to create a film of fluid thereon. Consequently, heat transfer to the cooling fluid is rendered easier. However, the cooling efficiency of jet impingement coolers is generally low due to the unstructured only flat impingement surface of the semiconductor chips.
Moreover, it is known to etch trenches into the backside of a silicon wafer which serve as microscopic fluid channels in the future semiconductor chips. This concept of micro-channel heat sinks is based on very short thermally conduction paths and a high surface enlargement factor and utilizes the good thermally conductivity of silicon. It has been shown that the highest cooling efficiencies are possible with channels having a width of 30 to 50 μm and a depth of 200 to 300 μm.
A drawback of this method is that surface enlargement of processor chips by deep trench etching of wafers causes yield reduction, either due to the deep trench etch process stability or due to mechanical fracture of the patterned surface. Therefore, the method is cost ineffective. Moreover, deep trench backside etching of wafers can cause process compatibility issue problems, does not allow a re-working of this process in case of defects and makes the semiconductor chips more fragile to breaking. As a consequence, such etching processes are not accepted in chip manufacturing.